Thermocouple Materials

Thermocouples are unsurpassed for making temperature-difference measurements. The thermoelectric power of thermocouple materials makes them adequate for use at hquid-air temperatures and above. At 20 K and below, the thermoelectric power drops to a few lV/K, and their use in this range is as much art as science. 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

The usual thermocouple materials are Pt and Pt-alloys. For increasing the rather low thermovoltage, thermoelements with pallaplat are very suitable, only however up to a maximum temperature of 1200 °C. 

For a detailed description of thermocouple materials, their maintenance, insulation, welding, etc., see also Wang (1990). 

Fig. 2. Law nf Intermediate Metals makes it possible to use foreign wires to connect thermocouple to measuring instrument. Thermocouple materials A and B can be connected to the instrument by use of connecting materials C and D. If the temperatures at Xj and X2 are both at T and if temperatures at Y and Y2 are both at Ti, the emf of the circuit will be independent of materials C and D...

Thermojunctions may be formed by welding, soldering or pressing the materials together. Such junctions give identical emfs (by law (iii)), but may well produce different currents as the contact resistance will differ depending on the joining process utilised. Whilst many materials exhibit thermoelectric effects, only a small number are employed in practice. The characteristics of the more common thermocouple materials are listed in Table 6.4. 

Thermocouples may be constructed of several different combinations of materials. The performance of a thermocouple material is generally determined by using that material with platinum. The most important factor to be considered when selecting a pair of materials is the "thermoelectric difference" between the two materials. A significant difference between the two materials will result in better thermocouple performance. Figure 4 illustrates the characteristics of the more commonly used materials when used with platinum. 

Figure 4 Thermocouple Material Characteristics When Used with Platinum...

A thermocouple is placed in a large heated duct to measure the temperature of the gas flowing through the duct. The duct walls are at 425°C, and the thermocouple indicates a temperature of 170°C. The heat-transfer coefficient from the gas to the thermocouple is 150 W/m2 °C. The emissivity of the thermocouple material is 0.43. What is the temperature of the gas ... 

The extremely unreactive, silvery-white element is used in jewelry plating and is part of anodic and thermocouple materials. However, compounds containing Rh in oxidation states from (VI) to (—1) can be produced, the most common being those of Rh(III). Developments in Rh chemistry since the late 1960s have focused on various catalytic processes that use Rh(I) complexes, particularly those containing phosphines such as PRj (R is an aryl or alkyl group) and/or CO, because such ligands stabilize this oxidation state. 

Thermocouple Materials. Thermoelectric properties of different materials can be represented by their respective Seebeck coefficients. Usually this is done with reference to Eq. 16.20, assuming a standard reference material (material B) and a standard reference temperature T0 (usually 0°C see Fig. 16.17). 

Thermocouples and thermocouple materials are normally supplied to meet the tolerances specified in the table for temperatures above 0°C. The same materials, however, may not fall within the tolerances given for temperatures below 0°C in the second section of the table. If materials are required to meet the tolerances stated for temperatures below 0°C the purchase order must so state. Selection of materials usually will be required. 

Some environmental limitations of the standard thermocouple materials compiled by ASTM [36] are reproduced in Table 16.11. The thermal EMF of standard thermoelements relative to platinum is shown in Fig. 16.20 [36]. Seebeck coefficients (first derivative of thermal EMF with respect to temperature) for each of the standard thermocouples as a function of temperature are tabulated in Table 16.12. 

G. W. Burns and M. G. Scroger, The Calibration of Thermocouples and Thermocouple Materials, NIST Special Publication 250-35, National Institute of Standards and Technology, 1989. 

Temperature sensing is based on a thermocouple. Hence, various pairs of metal-based filaments and yarns, listed in Table 9.2, were analysed with respect to their applicability as thermocouple materials. 

The second experimental setup in Fig. 3.5 eliminates the need to carry the thermocouple wires to the measuring instrument by changing from thermocouple material to normal conductance copper in a reference ice bath. In this arrangement all subsequent Cu/Cu junction potentials cancel. Such an arrangement is particularly useful if the reference bath is automated or a controlled counter-emf is used. 

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